Plasma processing of large area substrates (e.g., more than two meters by two meters in size) is needed for a cost-effective manufacturing of display screens, thin film photovoltaic panels, light filters for windows and other large area mass-market products. However, to produce a substrate with properties that are the same over nearly the entire area of the substrate (which is necessary for good product yields), the plasma involved in the processing of the substrate should have approximately constant properties over its full extent in the processing chamber.
When high frequency RF power (at frequencies of 13.56 MHz and above) is employed to excite the plasma, the electrical potentials on the electrode (i.e., powered electrode) for forming this plasma may have a quarter-wavelength on the same order of magnitude as the size of the electrode. Consequently, a substantial variation in the amplitude of the RF voltage and current may be present across the electrode, causing a non-constant power injection into the plasma along the length of the electrode. This results from the fact that RF currents propagate as electromagnetic waves on the surface of the metal electrode, in much the same way as a loaded transmission line. The electrical power may be absorbed into the plasma both capacitively and inductively due to shunt or induced currents in the plasma as EM waves propagate along the electrode surface. This variation in the amplitude of the RF voltage across an electrode has been shown to be substantial for parallel plate electrodes when the power is provided at a single contact point on an electrode that extends over the full area of the plasma. For linear electrodes, larger variations occur when power is provided at a single contact point on a linear electrode that is larger than a small fraction of a wavelength. Such non-uniformity in the RF voltage amplitude (and similarly in the RF current) often results in a significant non-uniformity of the process over the surface of the substrate, which is not desirable.